I wonder how much of that has to do with semantic drift on Elohim, i.e. by the time the oldest extant manuscripts were written, the figure was already considered singular despite retaining the noun plural morphology. The implication there would be that earlier (now lost) manuscripts may have had plural verb agreement for Elohim, or maybe simpler / more plausible, there was a time in the oral tradition where Elohim was still considered a plural figure and would have naturally gotten plural verbs.

I think the fact that the plural morphology exists on the noun at all suggests at least that the figure started as a collective.

Edit: probably also worth a mention that portions of Genesis (e.g. Garden of Eden) mirror portions of the Epic of Gilgamesh, a story which is overtly polytheistic.